Hard disks are typically made from polished metal substrates, or platters, such as NiP plated aluminum. After polishing, the substrate is coated with various thin layers of materials which include magnetic material, and are then used either singly or in stacks within a disk-drive housing.
Hard disks utilize read/write heads to transfer data to and from the disk according to well-known principles of magnetic recording. Unlike floppy disk drives, hard disk read/write heads are not in contact with the disk-drive surface. Rather, they "fly" or are supported a few millionths of an inch from the disk's surface on a thin layer of air produced by the rapidly spinning disk.
The mechanical interface between the head and recording media in disk drives is viewed by many as one of the most challenging aspects of magnetic recording technology. The amount of information that can be packed onto a disk is determined in part by the spacing between the read/write transducer and disk. A closer spacing produces sharper transitions, allowing the data transitions to be recorded much closer together. Accordingly, there is a continuing effort to reduce the spacing between the read/write head and disk, yet still maintain physical separation between the head and media. Such small spacing between the disk and head implies the desirability of a very smooth disk surface having excellent flatness. Accordingly, the texture of the recording media surface is one component which plays an important role in reliably achieving reduced head/disk spacings.
The close spacing between a head and disk results in both kinetic and static friction. Although when spinning the head and disk are separated by a layer of air, during spin down the proximity of the two relative to one another results in kinetic friction. Static friction, commonly referred to as "stiction", occurs when the disk is not rotating and the head contacts the uppermost peaks of the disk's surface at rest. Static friction is measured during a repeated series of contact start stop (CSS) tests to determine disk reliability and expected life. While low flying height is highly desirable and maximized by providing maximum smoothness, extremely smooth surfaces place more material of the disk surface in contact with the head when the disk is not rotating, thus greatly increasing static friction.
Static friction generally increases with increased humidity. Static friction also increases as the head and disk stationary contact time increases. These effects are particularly severe after the disk has experienced contact start-stop wear.
Table 1 summarizes the average static friction measured for various humidity and stationary contact times after 2000 contact start-stop (CSS) cycles of wear for a typical prior art disk. Measurements were made using a standard 75% thin film slider with about 5 grams loading force.
TABLE 1 ______________________________________ Static Friction Summary for Prior Art Disk ______________________________________ Number of CSS cycles 2000 2000 2000 2000 Relative Humidity (%) 20 80 80 80 Stationary Contact Time (minutes) 0 0 60 600 (before static friction measurement) Mean Static Friction (Newtons) 0.030 0.079 0.157 0.189 ______________________________________
As is apparent, the static friction coefficient for a typical prior art disk increases drastically at high humidity with long stationary contact times. In order to provide a reliable disk drive, low static friction is required under these conditions. It would be desirable to reduce this static friction while at the same time enabling a low flying height.
In evaluating potentially new and production-capable disk drive designs, such disks are subjected to a series of tests which include a glide test and an electrical defect test. To pass a glide test, the slider housing the read/write head must not hit any bump or other surface attribute of the media for a given flying height. The electrical defect test then provides a plethora of read/write cycles for that flying height, and determines the number of defects which occur over the entirety of a given disk surface.
It is desirable to develop hard disk drives and methods for producing such drives which minimize stiction, allow reduced flying height while producing acceptable yield in a glide test, and which minimize defects in the electrical defect test.